With the release of vSphere 6.5, VMware upped the game for vCenter High Availability (vCHA) and introduced an active/passive/witness cluster setup to provide a failover cluster for vCenter Server Appliances. The diagram below shows the architecture of the solution.
Deploying vCHA can be done in two modes - “Basic” and “Advanced”. You can use Basic mode if the vCenter you want to be HA is managing the hosts it resides on - in this scenario the wizard configures your vCenter and deploys the Passive and Witness nodes for you.